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Humans’ indelible stamp on Earth clear 5000 years ago

By Fred Pearce

Ancient humans may have made clever use of fire as a land-management tool

(Image: Pgiam/Getty)

When did humans stamp our footprint on the planet? The idea that we have entered a geological epoch defined by our very presence – the Anthropocene – is gaining traction, but exactly when did this epoch begin? After the first atom bomb went off? At the start of the industrial revolution in the mid-18th century? Or was it a lot earlier? A new study argues that the Anthropocene began with the rise of farming or even in Neolithic times, when we took to widespread burning of the bush to hunt animals.

In a reappraisal of humanity’s footprint, Erle Ellis, a geographer at the University of Maryland in Baltimore, and colleagues calculate that across much of the world – excluding the poles – at least a fifth of the land had been transformed by humans as early as 5000 years ago. By contrast, most previous studies conclude that this level of transformation in land use was only reached around 100 years ago. Ellis’s group also argues that this degree of land use would have released enough carbon dioxide to have warmed the local climate of the time.

Even though there were only a few tens of millions of us back then, nature was on the back foot because individuals needed far more land to sustain themselves than we do today, says Ellis. Thanks to more intensive farming methods, per-capita land use in western Europe now is only around a sixth what it was 2500 years ago, while in south-east Asia it is less than a tenth.

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“We often assume that early agriculturalists couldn’t alter the landscape much because they lacked the technology,” says co-author Steve Vavrus of the University of Wisconsin in Madison. “But their impact was great because they didn’t need to be as efficient as modern farmers.”

Filling all usable land

Ellis and Vavrus’s study compares conventional models of past land use, which have usually assumed that the amount of land each human needs has remained largely unchanged, with their own alternative, in which early humans “expanded to fill all usable land, and then intensified land use as population densities increased”. They present archaeological evidence that the real world fits their model.

Sediments in lake beds, for instance, show massive peaks in tiny particles of charcoal, starting 60,000 years ago. This coincided with hunters setting fires to catch animals by driving them in a specific direction, and using fire to stimulate the regrowth of grasslands on which their prey fed. Another example is the soils beneath modern tropical forests in both the Americas and Africa that were enriched with manure and charcoal put there by farmers at least 2500 years ago.

This prehistoric tinkering with the planet may even have altered the climate. Over thousands of years, the great clearances of forests and transformations of grasslands with fire and plough probably released enough carbon from trees and soils to raise atmospheric carbon dioxide levels by 20 or 30 parts per million, calculates co-author Jed Kaplan of the Swiss Federal Institute of Technology in Lausanne. That’s equivalent to a 10 per cent rise compared to what the CO2 level would otherwise have been – small in terms of its warming influence compared to the effect of today’s human emissions of around 120 parts per million, but possibly enough to have triggered local changes.

A typical human even 8000 years ago had a lifetime carbon footprint of about 30 tonnes of CO2. That was around one tonne a year, compared to per-capita annual emissions of 2 to 3 tonnes today.

Steve Jackson of the Southwest Climate Science Center of the US Geological Survey based in Tucson, Arizona, who was not involved in either study, says the new work underlines an “increasing recognition that human societies have shaped ecosystems at a much broader scale than presumed”.

Not pristine but resilient

This picture of early human domination of much of the planet calls into question some common environmental assumptions, says co-author Dorian Fuller of University College London. It suggests, for example, that there is little truly pristine nature anywhere. Most apparently virgin rainforests are far from virgin; many are recovering from past clearance. This may disappoint some nature lovers, but “it shows nature is resilient”, says Kaplan. Nature adapts to our activities better than we often think.

However, the new study does not give us grounds for rejecting environmental concerns, he says. Our current problems – especially what may be runaway climate change caused by burning fossil fuels – are real. At least the study’s findings do offer important lessons for fixing those problems.

One is not to get hung up about whether ecosystems are natural. Most likely they are not, but all deserve protection. Another is to recognise that, throughout the Anthropocene, technology has repeatedly been our trump card. Through its use, intensified agriculture has allowed world populations to soar. And while some civilisations, such as the Maya and early Mesopotamians, may have wiped themselves out through environmental mismanagement, others were smarter, the new analysis suggests.

Between 30,000 and 20,000 years ago, the tribes of Eurasia underwent a change in strategy now known as the “broad spectrum revolution”. Faced with a decline in big game, probably through overhunting, people started to diversify their food sources by hunting a wider range of smaller animals, sometimes managing wild herds of deer and gazelle to ensure they did not die out. They also learned how to grind, boil, ferment and roast food, which allowed them to eat a much greater variety of food, and to develop early techniques of farming, such as seed propagation.

It worked well and was, says Fuller, an early example of adopting a more sustainable lifestyle to cope with resource shortages. That is a lesson we could usefully adopt in the modern era.

And as for the onset of the Anthropocene? Ellis is reluctant to put a definite date on it but Fuller is happy to pinpoint it to around 3000 BC. “This is the point at which we are beginning to see rises in both carbon dioxide and methane; domesticated fauna start to become widespread on all the Old World continents, and we have the emergence of urbanism and larger scale metallurgy around this time or soon after,” he says.